1. Technical Field of the Invention
The present invention pertains to the field of devices which produce uniform diffuse light over a variety of geometries including straight rectangular light-guiding plates and curved light pipes.
2. Description of the Related Art
Many sources of light, whether they are electrical in nature, like lamps and lasers, or natural, like direct sunlight, produce light in a spatially concentrated form. This is especially true of laser light and sunlight at the focus of a lens, but it is also true of commercial lamps in general, like incandescent and fluorescent lamps, and it is notably true of halogen lamps. For many applications in lighting, such as for advertising and for use in rear face illumination of liquid crystal displays, a means for uniformly and efficiently diffusing the light has to be introduced in the path from the concentrated source to the user.
An example of such a diffuse light source is a light-box. The traditional approach to fabricating a light-box has been to place an array of fluorescent lamps behind a piece of strongly diffusing glass. As the lamps age, invariably one lamp becomes dimmer than the others and the brightness of the light-box becomes noticeably nonuniform. An additional drawback of such a light-box is that it is bulky, and, as a result, it cannot easily be put on a wall, certainly not in homes, and it does not meet the specifications of light weight and compactness which is required for a liquid crystal display. An additional drawback of the traditional approach to building a light-box is that the number of configurations is limited and, as a result, a light box is limited to a small number of applications.
An alternative approach to providing a diffuse light source, which has attracted growing interest lately, is based on the use of guided light in edge-lighted devices. U.S. Pat. No. 4,648,690 issued to Makoto Ohe on Mar. 10, 1987 and assigned to Mitsubishi Rayon Company, Ltd., discloses an edge-lighted device wherein light from one or more fluorescent lamps is coupled into the edges of a transparent slab comprised of, for example, acrylic plastic. By the process of total internal reflection the transparent slab guides the light between its top and bottom surfaces. The top surface of the transparent slab is covered by a layer of light-diffusing material which is a layer of alkyl methacrylate polymer into which particles of titanium dioxide (TiO.sub.2) powder have been mixed. Some of the light guided by the transparent slab is diffused by the TiO.sub.2 particles in the top layer, to provide a source of diffuse light.
As noted in the above-mentioned patent a problem with edge-lighted devices is that the brightness of the diffused light is nonuniform, specifically, the brightness is higher near the edge where the light is entering the transparent slab and drops off as the light propagates into the region away from this edge. In fact the above-mentioned patent discloses an additional diffusing layer to ameliorate the nonuniformity problem. Notwithstanding the use of this extra layer the apparatus of the above-mentioned patent does not resolve the problem of a gradual decrease of the injected light beam as it propagates down the transparent slab. The further the light beam propagates, the weaker the beam becomes, and the harder it becomes to extract from it diffuse light of the same brightness as one obtains near the edge of the transparent slab where light is injected. After a certain propagation length in the slab the light beam is so weak that it is no longer possible to extract diffuse light from it with the brightness level required to maintain the desired degree of uniformity in the diffuse light over the entire slab.
Another edge-lighted device is disclosed in U.S. Pat. No. 4,642,736, issued on Feb. 10, 1987, to Tokihiko Masuzawa, Yoshitaka Kageyama, and Norizou Tomita, and assigned to Mitsubishi Rayon Company, Ltd. This patent describes an attempt at achieving uniform illumination by using a plurality of light transmission plates set an angle to the face-plate of the display. The disclose edge-lighted device suffers from the drawback of complexity in design and manufacturing and would require very fine tuning for each particular application in order to achieve substantial uniformity in the brightness of the diffused light.
The structures described in the above-mentioned and other previous art exhibit a certain degree of complexity and have not satisfactorily resolved the issue of obtaining both brightness uniformity and high efficiency. This is due in large measure to the fact that they have not addressed the fundamental problem of extracting diffuse light of uniform brightness from a beam of light whose intensity steadily decreases as it propagates. Moreover they are designed for very specific applications requiring a well-defined geometry so that they cannot be applied in a general way to a wide variety of geometries in a straightforward fashion.